Influx of Competition Leaves Early Players—Such as Delaware, New Jersey and West Virginia—Struggling to Keep Bets Flowing

Racetrack casinos used to contribute as much as $240 million a year to Delaware's tax coffers. But as the Northeast becomes saturated with gambling venues, the state's casino revenue has tumbled, prompting a new industry request—for a tax break.

"It's a different world for the Delaware casinos," said Democratic Gov. Jack Markell, who supports reducing the tax burden on casinos by $20 million a year to help them...